from what little info the classic show has spoonfed us,its hard to tell
my guess would that that they are engeniered on the genitic level and grown in bulk batches in tanks or jars like the clone troopers from star wars

Oh... well, you see, the thing is... DAZ no longer offers the creeper, and I used to have the creeper... but well, it' like I don't have the creeper anymore... and I miss him terribly.... I mean, who am I gonna kiss on New Years eve?